[DevTools Dark Mode] Add missing css variable in dark mode
[DevTools Dark Mode] Add missing css variable in dark mode

Bug: 1170685
Change-Id: Ifcef0af57f72af18041da9aa797113e52267606f
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3074823
Reviewed-by: Kim-Anh Tran <a72f38a79b5fac6811022b8a7fb84830d856c9c9@chromium.org>
Reviewed-by: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>
Commit-Queue: Nancy Li <75d1eae50692fe1e75f263dbfd52feffc9e7984f@google.com>

ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend